ITRFtoGCRF {asteRisk}

Convert coordinates from ITRF to GCRF

Description

The ITRF (International Terrestrial Reference Frame) is an ECEF (Earth Centered, Earth Fixed) frame of reference, i.e., a non-inertial frame of reference where the origin is placed at the center of mass of Earth, and the frame rotates with respect to the stars to remain fixed with respect to the Earth surface as it rotates. The Z-axis extends along the true North as defined by the IERS reference pole, and the X-axis extends towards the intersection between the equator and the Greenwich meridian at any time.

The GCRF (Geocentric Celestial Reference Frame) frame of reference is an Earth-centered inertial coordinate frame, where the origin is also placed at the center of mass of Earth and the coordinate frame is fixed with respect to the stars (and therefore not fixed with respect to the Earth surface in its rotation). The X-axis is aligned with the mean equinox of Earth at 12:00 Terrestrial Time on the 1st of January, 2000, and the Z-axis is aligned with the Earth´s rotation axis.

This function requires the asteRiskData package, which can be installed by running install.packages('asteRiskData', repos='https://rafael-ayala.github.io/drat/')

Usage

ITRFtoGCRF(position_ITRF, velocity_ITRF, dateTime)

Arguments

position_ITRF

Vector with the X, Y and Z components of the position of an object in ITRF frame, in m.

velocity_ITRF

Vector with the X, Y and Z components of the velocity of an object in ITRF frame, in m/s.

dateTime

Date-time string with the date and time in UTC corresponding to the provided position and velocity vectors. This specifies the time for which the conversion from ITRF to GCRF coordinates will be performed. It is important to provide an accurate value, since the point over the surface of Earth to which a set of GCRF coordinates refers varies with time due to the motion of Earth.

Value

A list with two elements representing the position and velocity of the satellite in the GCRF (Earth-centered non-intertial) frame of reference. Position values are in m, and velocity values are in m/s. Each of the two elements contains three values, corresponding to the X, Y and Z components of position and velocity in this order.

References

https://celestrak.org/columns/v02n01/

Examples

if(requireNamespace("asteRiskData", quietly = TRUE)) {
# The following were the position and velocity of satellite MOLNIYA 1-83 
# the 25th of June, 2006 at 00:33:43 UTC in the ECEF frame (in m and m/s).

position_ITRF <- c(1.734019e+06, -1.510972e+07, 39.08228)
velocity_ITRF <- c(1468.832, -3962.464, 4007.039)

# Let´s convert them into the GCRF frame

coordinates_GCRF <- ITRFtoGCRF(position_ITRF, velocity_ITRF, "2006-06-25 00:33:43")
}